Solid-state P-31NMR study of octacalcium phosphate incorporated with succinate

Abstract

Octacalcium phosphate (OCP) is an important model compound in the study of biomineralization. The octacalcium phosphate-succinate (OCPS) compound is prepared and characterized by ³¹P solid-state NMR spectroscopy. Taking advantage of the fact that the crystal structures of OCP and OCPS are very similar, an NMR strategy based on the ³¹P homonuclear double-quantum spectroscopy is developed to assign all the peaks observed in the ³¹P magic-angle spinning spectrum of OCPS. On the basis of our experimental data, the molecular formula of OCPS is determined to be Ca_7.81(HPO₄)_1.82(PO₄)_3.61(succinate)_0.56·zH₂O, where z ≤ 0.5. We find that mainly the phosphorus species at the P5 site will be displaced when succinate ions are incorporated to form the OCPS lattice. The stability of OCPS is significantly higher than OCP with respect to the hydrolysis reaction at high pH conditions. We conclude that the hydration layer of OCP is playing the key role in the structural transformation of OCP.